The present invention is a polymer composition and method for improving the gas barrier performance of polymeric containers and films, and particularly containers for food and beverages which are molded from thermoplastic polyester polymers. More particularly, the invention is a polymer composition and method for reducing the permeability of gases through molded polymeric containers, sheets and films by incorporating into the polymer from which the container, sheet or film is formed an effective amount of a barrier-enhancing additive of the type described herein.
The addition of small amounts of molecular additives to a base polymer can result in antiplasticization of the polymer whereby the modulus of the polymer increases below its glass transition temperature and its barrier to gas permeability can improve. For example, Robeson describes the use of phenyl-2-naphthyl amine in polysulfone [Robeson, L. M.; Faucher, J. A., J. Polym. Sci., Part B 7, 35-40 (1969)] and various polychlorinated aromatic molecules in polycarbonate and in polyvinyl chloride [Robeson, L. M., Polym. Eng. Sci. 9, 277-81 (1969)]. Maeda and Paul [Maeda, Y.; Paul, D. R., J Polym. Sci., Part B: Polym. Phys. 25, 981-1003 (1987)] disclose the use of tricresyl phosphate in polyphenylene oxide to lower the sorption of carbon dioxide (and therefore its permeability). However, the need exists to improve the gas barrier performance of polymer resins of the type currently used for molded containers for food and beverages, and, in particular, poly(ethylene) terephthalate (PET) thermoplastic polyester polymers used for producing injection stretch blow molded bottles for packaging water, carbonated soft drinks and beer. Additives selected from 4-hydroxybenzoates and related molecules of the type described herein have not been suggested.
The present invention and the inventive features described herein reside in the discovery of certain barrier-enhancing additives for thermoplastic polymers. The invention is a polymer composition that contains one or more of the additives and a method for reducing gas permeability of shaped polymeric articles produced from such a composition, such articles being generally selected from containers, sheets and films.
The method comprises incorporating into the polymer an effective amount of a barrier-enhancing additive, or a mixture of barrier-enhancing additives, selected from the group consisting of:
(a) monoesters of hydroxybenzoic acid and hydroxynaphthoic acid of the formula (A) 
wherein R is C1-C8 alkyl, benzyl, phenyl or naphthyl; Ar is substituted or unsubstituted phenylene or naphthalene; or formula (AA) where M is a cation such as, but not limited to, sodium, ammonium, tetraalkyl ammonium, potassium, calcium, magnesium or zinc;
(b) diesters of hydroxybenzoic acid of the formula (B) 
wherein Ar is as defined above, and R1 is C1-C8 alkyl, (CH2CH2O)kCH2CH2 where k is 1 or more, benzyl, phenyl or naphthyl; or formula (BB) where M is as defined above.
(c) monoamides of hydroxybenzoic acid and hydroxynaphthoic acid of the formula (C) 
wherein R and Ar are as defined above; or formula (CC) where M is as defined above.
(d) diamides of hydroxybenzoic acid of the formula (D) 
wherein Ar is as defined above, and R2 is C1-C8 alkyl, (CH2CH2O)kCH2CH2 where k is 1 or more, benzyl, phenyl or naphthyl; or formula (DD) where M is as defined above.
(e) ester-amides of hydroxybenzoic acid of the formula (E) 
where Ar is as defined above and R3 is C1-C8 alkyl, C1-C8 dialkyl, (CH2CH2O)kCH2CH2 where k is 1 or greater, benzyl, phenyl or naphthyl, or formula (EE) where M is as defined above. As used herein, an effective amount, i.e., the preferred range of barrier enhancing additive, is from 0.1% by wt. to 20% by wt. of the base polymer comprising the polymeric article.
Polymeric articles, and particularly extruded film or injection stretch blow molded polyester (e.g., PET) bottles, which contain one or more of the barrier-enhancing additives described herein, exhibit substantially reduced oxygen and carbon dioxide permeability values when measured according to ASTM D3985 and water vapor permeability values when measured according to ASTM F1249 in comparison to corresponding polymeric articles which contained no barrier-enhancing additives.
The present invention resides in the discovery that oxygen, water vapor and carbon dioxide (CO2) permeability values for shaped polymeric containers and films can be substantially reduced by incorporating into the base polymer from which the articles are formed from about 0.1% by wt. up to about 20% by wt. of a barrier-enhancing additive of the type defined herein.
A uniform physical blend, or mixture, is prepared comprising the base polymer and one or more barrier-enhancing additives in the desired concentrations. As used herein with reference to the invention, the term xe2x80x9ccompositionxe2x80x9d is intended to mean a physical blend or mixture. Water-sensitive base polymers, such as, for example, polyesters should preferably be thoroughly dried by heating under air or nitrogen flow or vacuum as known to those experienced in the art. The mixture is then heated and extruded or molded at a sufficiently high temperature to melt the base polymer and provide for sufficient mixing of the additive or mixture of additives within the base polymer matrix. By way of example using PET, such melt temperature ranges from about 255xc2x0 C. to 300xc2x0 C. The composition thus produced comprises the barrier-enhancing additive (or mixture of such additives) substantially in its (their) original molecular form; that is, only small amounts of barrier-enhancing additive have been observed to react with the base polymer via trans-esterification or other reaction mechanism typical of the functional groups present. It is preferred to prepare and extrude or mold the polymer composition under conditions of relatively low temperature and processing residence time which thereby minimizes the opportunity for the barrier-enhancing additives to react with the base polymer. Best performance in terms of desirable mechanical properties of polymeric containers and films produced according to the invention is achieved when no more than about 10% of the gas barrier-enhancing additive has reacted with the base polymer. As a consequence of any reaction of a gas barrier-enhancing additive within the scope of the invention with a base polymer, the molecular weight of the starting base polymer may decrease.
The gas barrier-enhancing additives found to be most suitable for carrying out the invention are selected from the group consisting of:
(a) monoesters of hydroxybenzoic acid and hydroxynaphthoic acid of the formula (A) 
wherein R is C1-C8 alkyl, benzyl, phenyl or naphthyl; Ar is substituted or unsubstituted phenylene or naphthylene; or formula (AA) where M is a cation such as, but not limited to, sodium, ammonium, tetraalkyl ammonium, potassium, calcium, magnesium or zinc;
(b) diesters of hydroxybenzoic acid of the formula (B) 
wherein Ar is as defined above, and R1 is C1-C8 alkyl, (CH2CH2O)kCH2CH2 where k is 1 or more, benzyl, phenyl or naphthyl; or formula (BB) where M is as defined above.
(c) monoamides of hydroxybenzoic acid and hydroxynaphthoic acid of the formula (C) 
wherein R and Ar are as defined above; or formula (CC) where M is as defined above.
(d) diamides of hydroxybenzoic acid of the formula (D) 
wherein Ar is as defined above, and R2 is C1-C8 alkyl, (CH2CH2O)kCH2CH2 where k is 1 or more, benzyl, phenyl or naphthyl; or formula (DD) where M is as defined above.
(e) ester-amides of hydroxybenzoic acid of the formula (E) 
where Ar is as defined above and R3 is C1-C8 alkyl, C1-C8 dialkyl, (CH2CH2O)kCH2CH2, where k is 1 or greater, benzyl, phenyl or naphthyl, or formula (EE) where M is as defined above.
The above-defined barrier-enhancing additives can be obtained from commercial suppliers or they can be synthesized using established procedures.
Base polymers most suitable for use in practicing the invention comprise thermoplastic homopolymers, copolymers (both block and random), and blends of such thermoplastic polymers. Most suitable are polyester homopolymers and copolymers. Among suitable polyester base polymers are those polymers which contain structural units derived from one or more organic diacids (or their corresponding esters) selected from the group consisting of terephthalic acid, isophthalic acid, naphthalene dicarboxylic acids, hydroxybenzoic acids, hydroxynaphthoic acids, cyclohexane dicarboxylic acids, succinic acid, glutaric acid, adipic acid, sebacic acid, 1,12-dodecane dioic acid and the derivatives thereof, such as, for example, the dimethyl, diethyl, or dipropyl esters or acid chlorides of the dicarboxylic acids and one or more diols selected from ethylene glycol, 1,3-propane diol, nathphalene glycol, 1,2-propanediol, 1,2-, 1,3-, and 1,4-cyclohexane dimethanol, diethylene glycol, hydroquinone, 1,3-butane diol, 1,5-pentane diol, 1,6-hexane diol, triethylene glycol, resorcinol, and longer chain diols and polyols which are the reaction products of diols or polyols with alkylene oxides.
In a preferred embodiment of the invention the polyester base polymer is polyethylene terephthalate (PET), which includes PET polymer which has been modified with from about 2 mole % up to about 5 mole % of isophthalate units. Such modified PET is known as xe2x80x9cbottle gradexe2x80x9d resin and is available commercially as Melinar(copyright) Laser+polyethylene terephthalate brand resin (E. I. du Pont de Nemours and Company, Wilmington, Del.). As used hereinafter in illustrating the invention, the term PET will refer to commercially available xe2x80x9cbottle gradexe2x80x9d polyester resin.
Preparation of Film and Container Samples
Film samples are indicative of the improved gas barrier properties obtainable from the invention. Such film samples were generated from physical blends of a base polymer and a selected additive from among those described herein, and the samples were either compression molded or extrusion cast using a co-rotating twin screw extruder with a slit die, typically having a 0.38 mm gap, a quench roll, and a vacuum port on the front barrel section, with barrel, adapter, and die temperatures set at 240xc2x0 C. to 275xc2x0 C. depending on the polymer composition being used. Melt temperatures were measured with a thermocouple, and, for samples prepared using a twin screw extruder, melt temperatures were typically about 15xc2x0 C. to 20xc2x0 C. above the set temperature. In a few instances as noted, a transfer line, in which static mixers were installed within the line in place of a compounding screw, was used along with a slit die. Films were typically 0.05 to 0.25 mm thick. The thick films were subsequently stretched biaxially simultaneously to 3.5xc3x97 by 3.5xc3x97 using a Long stretcher at 90xc2x0 C., 9000%/minute unless otherwise noted.
For fabricating bottles, 26 g preforms were injection molded using a Nissei ASB 50 single stage injection stretch blow molding machine with barrel temperatures set at about 265xc2x0 C. and with a total cycle time of about 30 seconds. The preforms were immediately blown into 500 mL round-bottomed bottles with a blow time of 5 seconds. All other pressure, time and temperature set-points were typical for commercially available PET bottle resin.
Tensile bars xe2x85x9xe2x80x3 thick were molded using a 6 oz. injection molding machine with the following machine set-up: barrel temp: 255xc2x0 C., mold temp: 20xc2x0 C./20xc2x0 C., cycle time: 20 sec/20 sec, injection pressure: 5.5 MPa, RAM speed: fast, screw speed: 60 rpm, and back pressure: 345 kPa.
Analytical Procedures
NMR Spectrometry
Samples for 1H NMR were dissolved in tetrachloroethane-d2 at 130xc2x0 C. Spectra were acquired at 120xc2x0 C. at 500 MHz.
Thermal Analysis
Differential Scanning Calorimetric data were acquired at 2xc2x0/min on a TA Instruments calorimeter.
Permeability
Oxygen permeability values (OPV) were measured for each sample according to ASTM procedure D3985 at 30xc2x0 C., 50% RH on an Ox-Tran 1000 instrument from Modem Controls, Inc. Carbon dioxide permeability was measured at 25xc2x0 C. and 0% RH on a Permatran CIV instrument, also from Modern Controls, Inc. Water vapor permeability was measured at 37-38xc2x0 C., 100% RH on a Permatran-W600 instrument, also from Modem Controls, according to ASTM procedure F1249.
Intrinsic Viscosity
Intrinsic viscosity values were determined from 0.4 wt % solution of polymers or polymer blends in a 1:1 (by weight) mixture of methylene chloride and trifluoroacetic acid at 20xc2x0 C.